The present invention relates generally to ceramic materials and, more particularly, to sprayable phosphate cement coatings and a novel method and apparatus for producing them.
Ceramic cements are mixtures of water and reactive metal oxides that harden and fasten upon setting. Cements have a variety of familiar uses, such as the adhesive component to concrete (essentially an agglomeration of rocks held together by cement), the bonding layer that holds bricks together to form walls, as structural building materials such as patio or garage slabs. The cement of choice for most of these familiar uses is Portland cement, a mixture of water and calcined lime and silica. Upon curing, the primary constituents of Portland cement are dicalcium silicate and tri-calcium silicate phases. Portland cement has the advantage of being cheap to produce and relatively easy to mix and pour. Part of the reason Portland cement is so cheap is because the silica component may come from a wide variety of sources, usually silica-containing clays, and also because these clays do not have to be especially pure or consistent.
Portland cement also suffers from some disadvantages, inconsistency of physical properties arising from the inherent inconsistency of the source materials (both in composition and quality) being chief among them. Portland cements also have the disadvantage of having a relatively high viscosity. While they are well adapted to pouring and spreading, Portland cements are not well suited for pumping and spraying. Moreover, Portland cements are characterized by a relatively slow curing time. Another disadvantage of Portland cement is that it does not bond well to itself, especially if the existing cement surface is already hardened. Portland cement-containing structures, such as cement driveways or road segments, must be formed in essentially one step. If there is an interruption in the forming of a Portland cement body sufficient to allow the cement to begin to cure, a structural discontinuity or xe2x80x9ccold jointxe2x80x9d can result. Moreover, Portland cement cannot be used to patch a Portland cement structure absent costly and time consuming surface pre-treatment at the patch interface. While Portland cement is usually applied by pouring from a mixer or by spreading from a palette, it can also be sprayed. Sprayed Portland cement, or xe2x80x9cshotcretexe2x80x9d, is applied as a thick, rough layer of cement only in industrial applications that do not necessitate even or controlled coating, such as xe2x80x9cshotcretingxe2x80x9d over wire mesh for producing the foundations of swimming pools and for walls of tunnels and mines. Shotcrete is applied in very thick rough coats through enormous and expensive pneumatic sprayers and pumps that are not suited for smaller scale applications. Shotcrete sprayers cannot produce thin coatings or smooth finishes, and shotcreted surfaces sacrifice aesthetics for functionality. Portland cements set up and harden very slowly and are fairly porous, especially to road salt, which can degrade and rust steel reinforcement members in the concrete, causing expansion of the reinforcement members and the eventual rupture of the cement from within.
Another kind of cement is phosphate cement. Phosphate cements undergo an acid-base reaction during curing. Typically, the acid component is either phosphoric acid (usually in liquid form) or an alkali-earth phosphate salt such as magnesium phosphate, calcium phosphate or ammonium phosphate. The base component is typically dead burned magnesium oxide. The compositions of the acid and base pair are chosen such that the resulting combination will react to form a cementitious metal-phosphate. The acid and base components when mixed rapidly cure to form a cementitious metal phosphate phase. The phosphate cement forms by a highly exothermic reaction and sets up rapidly, quickly agglomerating and increasing in viscosity.
Most phosphate cements have excellent strength and hardness characteristics, and have the additional advantage of adhering to most other materials, including cement (both phosphate and Portland), brick, metal, wood, most wood products, insulation, asphalt, roofing materials, membranes and some glasses. Phosphate cements also have excellent chemical stability and compressive strength, and have toughness characteristics superior to those of Portland cement. Moreover, phosphate cements tend to set up with little or no open porosity and therefore can be used to form waterproof forms and seals. Phosphate cements, like most ceramics, are fireproof and tend to be electrically nonconductive and good thermal and acoustic insulators.
Traditionally, phosphate cements have been used almost exclusively for dental and biological applications, road patching, and specialized refractory applications. This is because phosphate cements are roughly an order of magnitude more expensive than Portland cement and cannot be used in bulk because the highly exothermic nature of the phosphate reaction causes phosphate cements to set up rapidly and to agglomerate, while generating a lot of heat. Unlike in Portland cement, where the heat of hydration evolves slowly and plateaus, the heat of hydration of phosphate cements spikes quickly, with great heat evolution occurring promptly after the cement is mixed. This results in the phosphate cement setting up too quickly to be workable. There are a variety of coating applications (fireproofing, water and fluid sealants, electrical insulation foam, electrical insulation coatings, thermal insulation coatings, chemical insulation coatings, rust proofing, overcoating existing roofs, walls, drywall, siding, floors, basements, roads and the like) that could be addressed by a thin or thick ceramic coating of a material having the properties of phosphate cement, but currently the technology does not exist to commercially apply thin cement coatings and, more particularly, to spray phosphate cements coatings. While the superior properties of phosphate cements would make them desirable for a much wider range of applications, their reactivity makes them ill-suited for bulk mixing, dipping, brushing, rolling and spraying since they tend to thicken and agglomerate quickly, rapidly clogging and packing spray nozzles, needle valves, hoses, and containers. This makes phosphate cements impractical for spraying, especially since most commercial spray apparati have orifices and conduits too small to accommodate the flow of a liquid having the density and viscosity of a phosphate cement. Further, most commercial spray apparati are expensive, and would be ruined by phosphate cements setting up in their hoses, nozzles, and containers, making their usage with phosphate cements impractical. Moreover, since ejecting the phosphate cement is the primary method of dissipating the excess waste heat generated by the acid-base reaction, a clogged spray line or nozzle can contribute to the overheating of the sprayer system, therefore increasing the hazard of fire or an explosion of the closed container. Further, overheating of the cement mixture in the sprayer also increases the reaction rate, thereby evolving even more heat and potentially causing further agglomeration in the spray gun and hoses resulting in a catastrophic runaway reaction.
There are currently no known cements capable of being applied as a thin, sprayed on coating or layer. There are also currently no known phosphate cement compositions that may be applied to a substrate by conventional spraying, coating, dipping or brushing techniques. There is therefore a need for a phosphate cement material with a controllably slow reaction and curing rate that can be mixed in bulk with a stable, low viscosity suitable for application as a thin coating via sprayer or via conventional application techniques. The present invention addresses this need.
One form of the present invention relates to a phosphate cement composition with a sufficiently controlled reaction rate that the phosphate cement may be mixed in bulk and with suitable viscosity to be sprayed. Another form of the present invention relates to a method and apparatus for mixing and spray applying a phosphate cement composition.
One object of the present invention is to provide an improved cement. Related objects and advantages will be apparent from the following description.